Graphene, an atomically thin layer of graphite, is attracting considerable interest in material science since the recent discovery of its appealing electronic properties such as high charge carrier mobility, ambipolar switching capability and the quantum Hall effect. The conjunction with its chemical robustness and superior mechanical properties makes graphene an ideal candidate for a number of applications that require ultrathin but yet stable and highly conductive layers or large specific areas as for instance in energy storage applications.
One of the major limitations for using graphene in efficient digital switching applications is the absence of an electronic band gap. This obstacle can be overcome by structuring graphene down to the nanometer scale where quantum confinement induces band gaps that are characteristic for a given shape and size of the graphene nanostructure. The most prominent examples using quantum confinement for the opening of a band gap are carbon nanotubes (CNTs), where periodic boundary conditions along the circumference are responsible for the existence of both, metallic and semiconducting CNTs, as well as armchair graphene nanoribbons (AGNRs), where confinement to a very narrow strip of graphene induces sizable band gaps if the width/diameter is limited to a few nanometers.
For both, CNTs, and AGNRs, the structural boundary conditions conserve the symmetry between the two atomic sublattices A and B (see FIG. 5c for AGNR).
A much richer diversity of electronic properties is predicted for graphene nanostructures where the edges break the symmetry between the A and B sublattices. The most prominent example are zigzag graphene nanoribbons (ZGNRs) where the atoms forming the two opposite edges belong to complementary sublattices (FIG. 5a). Electronic structure simulations reveal related localized edge states, which are magnetically coupled to each other [Fujita, M., et al., Journal of the Physical Society of Japan 65, 1920-1923 (1996);]. In the case of ZGNRs, for instance, the localized states belonging to the two opposed edges couple antiferromagnetically to each other and thus allow for an efficient spatial separation of spin up and spin down electrons to the opposite respective edges.
Based on these edge-related spin-polarized properties, computational simulations have been used to explore a number of specific edge configurations. Among the most appealing predictions for ZGNRs are spin-polarized charge carrier injection into graphene, half-metallic charge carrier properties (metallic properties for one spin component and semiconducting properties for the other) as well as giant magnetoresistance.
Cove type GNRs (CGNR) can be described as a special case of ZGNR in which carbon atoms have been added, respectively removed from the perfect zigzag edge resulting in the characteristic cove type structure elements. As in the case of ZGNR, in CGNR the localized states belonging to the two opposed edges couple antiferromagnetically to each other and allow for an efficient spatial separation of spin up and spin down electrons to the opposite respective edges.
Armchair-type, zigzag-type or cove-type edges are shown in the following formulae:
wherein    Ext is an edge substituent and stands for a substituent that is not part of the conjugated aromatic GNR structure network, e.g. a hydrogen atom;    Int is an atom which is part of the conjugated aromatic GNR structure network, e.g. a sp2 carbon atom). The positions of the double bonds are chosen arbitrarily since they together with the substituents Int form an extended conjugated system.
However, standard top-down fabrication techniques for the fabrication of GNR such as cutting graphene sheets e.g. using lithography, unzipping of carbon nanotubes (e.g. described in US 2010/0047154 and US2011/0097258), or using nanowires as a template are not suitable for ribbons narrower than 5-10 nm, because the edge configuration cannot be precisely controlled and they do not yield ribbons of monodisperse width. For high-efficiency electronic devices operating at ambient temperature, the ribbons need to be less than 10 nm wide, their width needs to be precisely controlled and, importantly, their edges need to be smooth because even minute deviations from the ideal edge shapes seriously degrade the electronic properties.
Due to the inherent limitations of lithographic methods and of other known approaches to fabricate graphene nanostructures, however, the experimental realization of GNRs with controlled zigzag and/or cove type edge structures in the hexagonal sp2 carbon network with the required high precision has remained elusive. Bottom-up approaches based on cyclodehydrogenation reactions in solution (see e.g. WO 2012/149257, KR 101082335 B, WO 2013/061256) or on solid substrates (see e.g. WO 2012/145101, WO 2013/072292) have recently emerged as promising routes to the synthesis of nanoribbons and nanographenes with precisely controlled structures.
At least two general types of precisely controlled linear nanoribbon structures can be distinguished. In a first type, the edges are forming a straight line along the nanoribbon, while in another type, sometimes called ‘chevron’ type or ‘nanowiggles’ (described e.g. in Phys. Rev. Lett. 2011 (107), 135501 or in J. Am. Chem. Soc. 2012 (134), 18169), the edges are lying on a corrugated or saw-toothed line. The latter case can also be described as a periodic repetition of alternatingly aligned graphitic nanoribbon subunits seamlessly stitched together without structural defects.
The edges of the graphene nanoribbons may be terminated either with hydrogen atoms and/or with any other organic or inorganic groups.
For solution-based approaches using oligo phenylene precursors a polymer is typically prepared in a first step which is subsequently converted into the graphitic structure by Scholl-type oxidative cyclodehydrogenation. All of the reported solution based methods yield graphene nanoribbons with exclusively armchair type edge carbon atoms (with exception of both ends of the GNR) or armchair type edge carbon atoms and cove type edge carbon atoms (with exception of both ends of the GNR), whereby in the latter case the proportion of unambiguously assignable cove type edge carbon atoms is less than 20% of the sum of all edge carbon atoms.
The surface-confined bottom-up approach to controlled graphene nanoribbons as described in Nature 466, pp. 470-473 (2010), WO 2012/145101, and WO 2013/072292, typically results in armchair graphene nanoribbons. No graphene nanoribbons that do contain zigzag type edge carbon atoms and only graphene nanoribbons in which the proportion of unambiguously assignable cove type edge carbon atoms (with exception of both ends of the GNR) is less than 20% of the sum of all edge carbon atoms have been obtained.
It is an object of the present invention to provide a graphene nanoribbon (GNR) containing zig-zag type edge carbon atoms, cove type edge carbon atoms or a combination thereof in positions which are not at the end of the GNR, wherein the position of zigzag type edge carbon atoms and cove type edge carbon atoms and the distance between zigzag type edge carbon atoms and cove type edge carbon atoms as well as the ratio of zigzag type edge carbons to cove type edge carbons and to armchair carbons is precisely controlled. A further object of this invention is a process for preparing such a graphene nanoribbon.